What should I do about this?

I saw a chiropractor for back pain and he said that muscles tightened up and that's why it hurts so bad. Then he said to limit sitting. I have to go back to school on Monday (this week is midterms and I don't have any), which means that I have to sit for 7 hours getting up only to change classes and go to gym but I don't know what I should do because I am supposed to limit sitting until it is better.